With the development of micro-electro-mechanical system (MEMS) technology, micro-size, reliable, and low-cost silicon accelerometers have been in volume production and widely adopted in automotive, consumer electronics, and industrial applications.
The operation principle of the differential capacitive micro-accelerometer is as follows. A proof mass is suspended above the substrate by spring beams/tethers and deflects in the plant of the substrate in the present of an acceleration field. When the proof mass moves, the gap between capacitors would decrease on one side while increasing on the other, resulting in a differential capacitance variation for measurement of acceleration.
To detect an out-of-plane (Z-axis) acceleration, the capacitive accelerometer is usually designed with torsional parallel-plates. Since the plate weight (proof mass) is asymmetrically distributed with respect to the rotational axis, acceleration along the normal axis to the substrate will make the top plate rock in one direction or another and thus cause differential change of capacitance on the two sides of the rotational axis.
However, in the conventional design of Z-axis accelerometer, a significant part of the poof mass to the far tip cannot be utilized for sensing, but only for creating imbalanced motion in response to inertial force, and the unused sensing area would have the largest signal displacement. This substantially limits the die area efficiency for electrical sensitivity.